1. Field of the Invention
The present invention relates to a reflection detection apparatus that detects a reflected light from an object and that is suitable for detection of a color shift and a density in a color image forming apparatus such as a color laser printer or a color copier.
2. Description of the Related Art
Of the above-described color image forming apparatuses, a tandem-type image forming apparatus provided with photoreceptors for multiple colors, for example, forms a pattern on an intermediate transfer belt and detects, by using a light-receiving sensor (reflection sensor), a reflected light from the pattern to perform detection of the color shift and density as described below.
In the color shift detection, the tandem-type image forming apparatus uses a color shift detection-purpose image composed of a reference color pattern in which portions of a reference color are cyclically formed and a comparison color pattern in which portions of a comparison color are formed between the reference color portions. The apparatus detects a time at which each color pattern passes above the reflection sensor in response to changes in light amount of a specularly reflected light which is generated by the passage of each color pattern and calculates, from a result of the detection, a color shift amount of the comparison color with respect to the reference color.
On the other hand, in the density detection, the apparatus uses a density-inspection-purpose image (dither pattern) constituted by a toner image expressed by area coverage modulation. The apparatus detects, by the reflection sensor, a light amount of the specularly reflected light from a surface of the intermediate transfer belt on which the toner image is not formed and calculates the density by using a decrease amount of the light amount of the specularly reflected light from the surface of the intermediate transfer belt on which a toner image is formed depending on a tone corresponding to a result of the detection.
In this process, a diffusely reflected light from the toner mixes with the specularly reflected light, which causes an error in detected value of the light amount of the specularly reflected light. For this reason, subtracting a detected light amount of only the diffusely reflected light from a detected light amount of a total reflected light, namely, the mixture of the specularly and diffusely reflected lights enables extracting only the light amount of the specularly reflected light, which enables calculating an exact density. However, in the specularly reflected light from the intermediate transfer belt, a ray fluctuation is generated corresponding to a relative angle that depends on a relative alignment relation between the intermediate transfer belt and the reflection sensor. A large ray fluctuation causes the reflection sensor to erroneously detect the specularly reflected light when only the light amount of the diffusely reflected light is to be detected. This erroneous detection consequently makes it impossible to detect an exact light amount of only the specularly reflected light, namely, the density.
Japanese Patent Laid-Open No. 02-256076 discloses a reflection sensor having a light-receiving region whose area is increased so as to prevent the specularly reflected light from reaching outside of a light-receiving region of the reflection sensor even if the ray fluctuation is generated.
However, such an increase in light-receiving region area for the specularly reflected light like the reflection sensor disclosed in Japanese Patent Laid-Open No. 02-256076 results in an increase in light amount of the diffusely reflected light reaching the light-receiving region from toner. This increase in the light amount of the diffusely reflected light reaching the light-receiving region increases, in density detection, the light amount of the diffusely reflected light compared to that of the specularly reflected light which decreases with an increase of the density, which makes it impossible to acquire, from the reflection sensor, a density detection signal with a sufficient amplitude corresponding to the light amount if the specularly reflected light. In this case, the light amount of the specularly reflected light from the surface of the intermediate transfer belt on which the toner image is not formed, the light amount being a reference for the density, contains an imaging noise component due to reflectance unevenness on the surface and to minute concavities and convexities caused in manufacturing of the intermediate transfer belt. For this reason, a decrease in amplitude of the density detection signal decreases an S/N ratio of the density detection signal.